In the process of fabricating semiconductor devices, each wafer needs to be etched for several times. A wafer-processing apparatus is used for etching wafers therein. Please refer to FIG. 1 schematically showing a conventional wafer-processing apparatus. An etching-solution container 11 is used for containing an etching solution, such as a buffer oxide etcher (BOE) and a buffer hydrofluoric acid solution (BHF). A gas-supplying pipe 14 is used for supplying a gas, such as an inert gas or nitrogen gas (N.sub.2), into said etching-solution container 11. Because the pressure of the inert gas contained in the gas-supplying pipe 14 is relative to a level of the etching solution, the level change of the etching solution can be controlled by detecting the pressure change of the inert gas. A pressure-leveling sensor 13 is used for detecting the pressure change of gas contained in the gas-supplying pipe 14 and correspondingly outputting a level signal to a controller 12. There are three kinds of level signals. A FIX level (FL) signal represents that the level of the etching solution is at a suitable level, a LOW level (LL) signal represents that the level of the etching solution is lower than the suitable level, and a HIGH level (HL) signal represents that the level of the etching solution is higher than the suitable level. Therefore, the controller 12 can control the level of the etching solution contained in he etching-solution container 11 according to the level signal.
The etching solution needs to be renewed after a predetermined amount of wafers has been etched by the etching solution because the etching solution is too dirty to etch more wafers. FIG. 2 is a flowchart showing the etching-solution-renewing process of the conventional wafer-processing apparatus. After the predetermined amount of wafers has been etched by the etching solution, the controller 12 outputs a discharge signal to the discharge valve 15 so as to have the discharge valve 15 discharge the etching solution from the etching solution container 11. Meanwhile, after the level of the etching solution is dropped below the suitable level, the pressure-leveling sensor 13 will output an LL signal to the controller 12. When the controller 12 receives the LL signal, the controller 12 outputs a close signal to close the discharge valve 15 after a short time to ensure that the etching solution is discharged completely. Then, the controller 12 will output a charge signal to the charge valve 16 so as to have the charge valve 16 charge the etching solution into the etching-solution container 11. The charge valve 16 will be closed by the controller 12 until a FL signal is outputted from the pressure-leveling sensor 13.
However, the conventional wafer-processing apparatus has some drawbacks. For the cleaning aspect, the gas supplied from the gas-supplying pipe 14 has been purposely dried in advance to eliminate the damp. Therefore, the dried gas will absorb a lot of damp from the etching solution when the dried gas is supplied into the etching-solution container 11, and the solutes contained in the damp will be dissolved out after water evaporates. These particles will usually adhere to the inner surface of the gas-supplying pipe 14. When the particles adhering onto the wall becomes very thick, the gas-supplying pipe 14 will be choked up and the pressure-leveling sensor 13 will not detect the pressure change of the gas precisely so that the wafer-processing apparatus needs to be stopped to clean the choked gas-supplying pipe 14. The cleaning process of the conventional wafer-processing apparatus is very time-consuming and etching-solution-wasting. In addition, when the cleaning process is done by men, it is very dangerous for the machine keepers to expose to such a harmful environment.